The present invention relates to methods and instruments for performing an interbody fusion of a disc space between two adjacent vertebrae. Specifically, the invention concerns laparoscopic techniques and instruments to prepare a fusion site and to insert fusion devices and implants.
The number of spinal surgeries to correct the causes of low back pain has steadily increased over the last several years. Most often, low back pain originates from damage or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be suffering from a variety of degenerative conditions, so that in either case the anatomical function of the spinal disc is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for the annulus, by way of a discectomy procedure. Since the damaged disc material has been removed, something must be positioned within the intradiscal space, otherwise the space may collapse resulting in damage to the nerves extending along the spinal column.
The intradiscal space is often filled with bone or a bone substitute in order to prevent disc space collapse and to promote fusion of the two adjacent vertebrae. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae, and the spine column was stabilized by way of a plate or a rod spanning the affected vertebrae. Once fusion occurred the hardware used to maintain the stability of the segment became superfluous. Moreover, the: surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
It was therefore determined that a more optimal solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, preferably with the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intradiscal implant that could be used to replace a damaged disc and maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. These "interbody fusion devices" have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intradiscal space and the vertebral end plates. These types of devices are represented by the patents to Brantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.
Interbody fusion devices can be generally divided into two basic categories, namely solid implants and implants that are designed to permit bone ingrowth. Solid implants are represented by U.S. Pat. Nos. 4,878,915; 4,743,256; 4,349,921 and 4,714,469. The remaining patents discussed above include some aspect that permits bone to grow across the implant. It has been found that devices that promote natural bone ingrowth achieve a more rapid and stable arthrodesis. The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with autologous bone prior to insertion into the intradiscal space. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone or bone substitute within the implant. In preparing the intradiscal space, the end plates are preferably reduced to bleeding bone to facilitate this tissue ingrowth. During fusion, the metal structure provided by the Michelson implant helps maintain the patency and stability of the motion segment to be fused. In addition, once arthrodesis occurs, the implant itself serves as a sort of anchor for the solid bony mass.
Another interbody fusion device that is designed to permit bone ingrowth is shown in FIG. 1. This device is described and claimed in co-pending parent application Ser. No. 08/411,017, filed on Mar. 27, 1995, which disclosure is incorporated herein by reference. In one embodiment, this invention contemplates a hollow threaded interbody fusion device 10 configured to restore the normal angular relation between adjacent vertebrae. In particular, the device 10 as shown in FIG. 1 includes an elongated body 11, tapered along substantially its entire length, defining a hollow interior 15 and having a largest outer diameter at the anterior end 12 of the device to receive the bone growth material. The body 11 includes an outer surface 16 with opposite tapered cylindrical portions and a pair of opposite flat tapered side surfaces 22 between the cylindrical portions. Thus, at an end view, the fusion device gives the appearance of a cylindrical body in which the sides of the body have been truncated along a chord of the body's diameter.
The cylindrical portions include threads 18 for controlled insertion and engagement into the end plates of the adjacent vertebrae. A started thread 19 is provided at the posterior end 13 of the device 10 to facilitate engagement within a prepared bore. The outer surface of this fusion device is tapered along its length at an angle corresponding, in one embodiment, to the normal lordotic angle of the lower lumbar vertebrae. The outer surface is also provided with a number of vascularization openings 24, 25 defined in the flat side surfaces, and a pair of opposite elongated bone ingrowth slots 27 defined in the cylindrical portions.
Various surgical methods have been devised for the implantation of fusion devices into a subject disc space. A patent to Dr. Gary Michelson, U.S. Pat. No. 5,484,437, discloses one such technique and the associated instruments. As described in more detail in that patent, the surgical technique involved the use of a hollow sleeve having teeth at one end that are driven into the adjacent vertebrae. These teeth and the sleeve maintain the disc space height during the subsequent steps of the procedure. In accordance with one aspect of the invention in the '437 Patent, a drill is passed through the hollow sleeve to remove the disc and bone material to produce a prepared bore for the fusion device. The drill is then removed from the sleeve and the fusion device is positioned within the disc space using an insertion tool.
In another aspect of the procedure and instruments disclosed in the '437 Patent, a long distractor is provided having penetrating portions that urge the vertebral bodies apart to facilitate the introduction of the necessary instruments. The long distractor can act as a guide for drilling and reaming tools concentrically advanced over the outside of the distractor to prepare the site for the fusion device.
While the Michelson technique represents a significant advance over prior surgical procedures for the preparation and insertion of fusion devices, the need for improvement remains. In particular, procedures and instruments that preserve the integrity of the surgical site are desirable. The present invention is directed to this need in the field.